Mechanistic Analysis of Alkyne Haloboration: A DFT, MP2, and DLPNO-CCSD(T) Study

Stereocontrol of the alkyne haloboration reaction has received attention in many experimental but few theoretical studies. Here we present a detailed quantum-chemical study of mechanisms leading to Z versus E isomers of haloboration products, considering acetylene and propyne combined with BCl3, BBr3, and BI3. Calculations using B3LYP-D3, MP2, and DLPNO-CCSD(T) methods are used to study polar reactions between the alkyne and BX3 in the absence and presence of an additional halide anion whose content in the reaction mixture can be controlled experimentally. The formation of anti-haloboration products via radical mechanisms is also explored, namely, by adding BX3 to (Z)-halovinyl radical. For the anti-haloboration of propyne, the radical route is prohibited by the regiochemistry of the initiating halopropenyl radical, while the polar route is unlikely due to a competitive allene generation. In contrast, energetically accessible routes exist for both syn- and anti-bromoboration of acetylene; hence, careful control of reaction conditions is necessary to steer the stereochemical outcome. Methodologically, MP2 results correspond better to the DLPNO-CCSD(T) energies than the B3LYP-D3 results in terms of both reaction barrier heights and relative ordering of energetically close stationary points.


INTRODUCTION
Haloboration is the addition of a boron−halogen bond across an unsaturated moiety. Its importance in organic chemistry stems from the fact that it introduces two highly valuable groups in the same step: a halide and a boron unit. 1 Extensive studies by Suzuki 2 demonstrated the usefulness of haloboration in organic syntheses due to its regio-and stereoselectivity. Experimental studies report haloboration adducts bearing the halogen at the more substituted carbon. 3 The first experimental report on haloboration is from 1964 by Lappert and Prokai, who studied a variety of alkynes in combination with several substituted boranes, including BBr 3 and BCl 3 . 4 In terms of reactivity, terminal alkynes undergo haloboration much more easily than internal alkynes, and BBr 3 facilitates the transformation with respect to BCl 3 .
Regarding stereochemical behavior, higher alkyne haloboration with either BBr 3 or BCl 3 led to the Z adduct at low temperature and to a mixture of Z and E adducts at high temperature. However, in the case of acetylene, the reaction provided exclusively an E adduct. Lappert and Prokai suggested two possible explanations. The first assumed that the mechanism of the addition to acetylene is different and cannot involve a four-center transition state (TS) (Scheme 1) as proposed for higher alkynes. The second one suggested kinetic reaction control for higher alkynes but thermodynamic control for acetylene.
In 1973, a detailed study by Blackborow reported on the bromoboration of phenylacetylene and hex-1-yne with BBr 3 , resulting in mixtures of Z and E adducts. 5 From the relative proportion of stereoisomers as a function of reaction conditions, the author concluded that different kinetically controlled mechanisms lead to the formation of the different stereoisomers. The E adduct formation was expected to be preferred in solvents more efficiently stabilizing polar transition states. However, in 1986 Wrackmeyer studied reaction of hex-3-yne with BBr 3 and showed that the Z/E ratio could develop over several days, indicating a slow Z to E isomerization of bromoboration adducts. 6 Similar findings were obtained for iodoboration of hex-3-yne by Siebert et al., who observed rapid formation of the Z adduct, which underwent slow isomerization to the E counterpart. 7,8 On the other hand, in their work on chloroboration of propyne and but-1-yne at −78°C, the same group reported E adduct formation without any reference to an isomerization step. 3 Despite the rich variety of stereochemical behavior depending on reactant identity and conditions, the first mechanistic study of haloboration was done only in 2012 by Wang and Uchiyama. 9 The latter study suggested that the reaction stereoselectivity could be understood via a mechanism opened by a syn addition followed by a steric Z/E conversion. Both steps were characterized by a four-center transition state involving a C− C multiple bond and a B−X single bond. The authors concluded, among others, that BBr 3 can catalyze the isomerization of the primarily formed Z adduct. The free energy barrier found for such isomerization was quite high (30.0 kcal/mol) but comparable to that of syn addition (25.9 kcal/mol) and still lower than the barrier of a thermal process (54.6 kcal/mol). Interestingly, a mechanism that would involve successive addition and elimination of hydrogen bromide was discarded in that study on the basis of the result obtained for hydrogen chloride in analogous chloroboration. 9 Recently, the mechanism of the acetylene bromoboration reaction has been reinvestigated in our group. Motivation for doing so stemmed from a 1994 work by one of us (C.M.) and M. Vaultier, who developed a highly stereoselective synthesis of 1-(dialkoxyboryl)-1,3-dienes based on acetylene bromoboration. 10 The origin of the stereocontrol was explored in a joint experimental and theoretical study whose aim was to identify both polar and radical mechanisms compatible with acetylene bromoboration outcomes as a function of reaction conditions. 11,12 In the first step of the isomerization mechanism introduced in our previous work, a bromine radical attacks the double bond of the Z adduct at the "boron end". The bromine radical can come from homolysis of HBr that seems to be an inevitable impurity in BBr 3 or from reaction of BBr 3 with oxygen, traces of which are capable of promoting radical chain pathways in reactions of many boron derivatives. 13−15 The addition of bromine radical is followed by rotation about the C−C single bond by ca. 180°, and the last step of the isomerization is the release of Br radical from the carbon at the "boron end" to form the E adduct. 16 In addition, our results indicated that apart from isomerization of the Z adduct, two alternative routes to the E adduct exist, namely, radical and polar direct anti addition pathways. In the first step of the radical reaction mechanism, Br radical reacts with acetylene to get a bromovinyl radical that attacks the BBr 3 molecule, affording a radical intermediate. It affords the E adduct after release of a bromine radical from boron. The first step of the polar reaction mechanism consists in BBr 3 addition to the acetylene multiple bond to give a zwitterion, whose positively charged carbon is in the second step attacked by Br − anion from experimentally inherently present HBr, affording an intermediate that gives the E adduct after release of Br − anion from boron. 12 In our previous work, other halogens and propyne as a model higher terminal alkyne were not studied, and in the work of Wang and Uchiyama, they were studied only to a limited extent, i.e., in the context of isomerization using the second BX 3 molecule and a four-center TS. In the current work, we explore polar and radical mechanisms leading to Z and/or E adducts in extended scopes. In terms of reactants, interactions of acetylene and propyne with BCl 3 , BBr 3 , and BI 3 are compared. The goal of this work is a comparison of thermodynamic (Δ r G) and kinetic (ΔG ⧧ ) characteristics of addition reactions for acetylene and propyne in combination with different haloboranes. This will give more general insight into the relative reactivity and the stereochemical behavior.
The paper is organized as follows: we first discuss polar additions to acetylene, second polar additions to propyne, and finally the respective radical mechanisms. Discussions concerning polar additions to both acetylene and propyne are further divided into parts concerning the syn additions and the halide-anion-catalyzed anti additions. Throughout this work, DLPNO-CCSD(T)/cc-pV5Z-PP//MP2/6-31+G*_SVP energy profiles are presented in the main text. MP2 and B3LYP-D3 Gibbs free energy profiles with the 6-31+G*_SVP and def2TZVPP bases are given in the Supporting Information. Benchmarking of methods and basis sets is discussed in section 2.2.

Computational Details.
The starting structures of BX 3 (X = Cl, Br, I), acetylene, propyne, and van der Waals complexes were built in the program Avogadro. 17 For these structures, geometry optimization was performed using the methods specified below. The guesses of all transition states were estimated using the single coordinate driving (SCD) method. These guesses were then optimized, followed by the frequency analysis. To verify the optimized transition states, intrinsic reaction coordinate (IRC) calculations followed by optimizations of local minima were done.
Structures were optimized at the MP2 18−23 level of theory as implemented in Gaussian 09, revision D.01, 24  The GD3BJ dispersion correction of Grimme 34 was employed in all DFT calculations, below denoted as B3LYP-D3. For Br and I, SVP basis sets of Ahlrichs 35,36 were used in combination with the 6-31+G* basis set of Pople 37−44 for H, B, C, and Cl (further labeled as 6-31+G*_SVP basis set). Additionally, MP2 calculations were performed with the Def2TZVPP extended basis set, 45,46 while B3LYP-D3 allelectron results for iodine compounds were compared with the small-core ECP46MWB pseudopotential approach 47 with the corresponding recommended orbital basis set of DZP quality.
The DLPNO-CCSD(T) single-point calculations were performed with the cc-pV5Z orbital basis and cc-pV5Z/C auxiliary basis of Dunning et al., 48−50 with the exception of iodine atoms in iodo derivatives. For the latter, the cc-pV5Z-PP orbital basis and cc-pVQZ-PP/C auxiliary basis of Dunning et al. along with the SK-MCDHF-RSC pseudopotential 51 were The Journal of Physical Chemistry A pubs.acs.org/JPCA Article emloyed. This basis set combination was used in order to overcome SCF convergence problems. Implicit SCRF modeling of solvation using CH 2 Cl 2 was employed in all calculations with the "pcm" (Gaussian) or "cpcm" option (ORCA).

Method Performance.
As suggested by a reviewer, our original approach (MP2 level for structures and energies) was replaced by a combined methodology: single-point DLPNO-CCSD(T)/cc-pV5Z electronic energy calculations were performed using stationary-point geometries, zero-point energy (ZPE), and thermal corrections from the MP2/6-31G*_SVP method. The DLPNO-CCSD(T) method is known to provide highly accurate reaction barriers, with its applicability being independent of the nature of the reaction. 52 Using the relatively big cc-pV5Z basis set renders the BSSE error negligible, and essentially "complete basis set" energies can be achieved. The method is, however, too demanding for wide-scope potential energy surface (PES) explorations. Instead, MP2 or B3LYP-D3 approaches are conventionally used to provide lower-level structures for higher-level energy evaluations.
Key decisions thus regarded the MP2 or B3LYP-D3 method choice for local minima and transition state structure calculations. Direct comparison of MP2 or B3LYP-D3 versus CCSD(T) structures was not possible since analytic gradients, necessary for efficient geometry optimizations, are not available for the DLPNO-CCSD(T) method in ORCA. We thus adopted an indirect approach comparing MP2 or B3LYP-D3 versus CCSD(T) total activation barriers and relative energies of intermediates. This is done in Figures S1−S6. In all of the latter, MP2 energy profiles�compared to B3LYP profiles� follow better the CCSD(T) energy differences between the reactants and the transition states. Figure S5 reveals the highest methodological (MP2 or B3LYP-D3) sensitivity of the reaction barriers. MP2 results, in the sense of highest TS energy with respect to reactants, are overestimated by at most 3 kcal/mol, while B3LYP-D3 results are underestimated by up to 7 kcal/mol with respect to the DLPNO-CCSD(T) reference. Furthermore, MP2 predicts in agreement with CCSD(T) a single-step profile for the chloro derivative, while B3LYP-D3 predicts the existence of an intermediate. Based on these results, we consider the MP2 method superior to the B3LYP-D3 one and employ it throughout this work for structure, ZPE, and thermal correction calculations.
Regarding basis set choice for MP2 structure determination, the combined 6-31+G*_SVP basis motivated by the study of Wang and Uchyiama 9 is compared with the larger Def2TZVPP basis in Figures S1 and S3. The smaller 6-31+G*_SVP basis overestimates the MP2 reaction barriers by at most 3 kcal/mol (in Figures S1a) and is thus considered a suitable compromise between price and accuracy for geometry determination.

Polar Additions to Acetylene. 3.1.1. syn-Halobora-
tion. An uncatalyzed syn-haloboration of acetylene is an electrophilic addition whose preliminary stage is a π-hole−πelectron interaction between boron and C 2 H 2 . 53 The resulting loose van der Waals complex was chosen as the starting point of the reaction energy profile in Figure 1. In general, the total energy barrier decreases with increasing halogen proton number, in agreement with the increasing Lewis acidity of BX 3 . 54 The reaction proceeds for BCl 3 and BBr 3 toward the  product in a single step through a four-center TS (cf. Figure  1a,b). For BI 3 , a weakly bonded π-adduct intermediate (IM) was localized at the MP2 (not B3LYP-D3) level (cf. Figure  S1), in agreement with the results of Wang and Uchyiama. 9 The DLPNO-CCSD(T) results on MP2 geometries presented here, however, confirm that even for BI 3 , syn-haloboration proceeds in a single step.

anti-Haloboration
Catalyzed by X − . Figure 2 displays reaction profiles for the opening steps of acetylene anti-haloboration in the presence of an X − catalyst. Formally, it is a termolecular reaction. However, since BX 3 is used as the solvent (i.e., BX 3 is in excess), the reaction is effectively a bimolecular process. The mechanism starts with the formation of the zwitterionic intermediate IM1.
First, BX 3 adds to the triple bond to give a zwitterion whose positively charged carbon is attacked by the halide anion. The zwitterionic structure represents a shallow local minimum on the PES for X = I, and the total activation barrier decreases from 16 kcal/mol (X = Cl) to 10 kcal/mol (X = I). In the second step, halide anion binds to the positively charged carbon to give intermediate IM2 characterized by a quaternary boron atom. Because of steric hindrance and electrostatic repulsion, X − approaches from the opposite side of acetylene than does the BX 3 molecule. This results in the E configuration at the double bond of IM2 within a single-step reaction.
In the last step, quaternary boron releases X − upon assistance of another molecule of BX 3 to give the E adduct and BX 4 − . 55,56 Our results for the corresponding Gibbs free energy profiles are shown in Figure 3, and the complete mechanism is summarized in Scheme 2. Explicit solvation of X − with BX 3 is essential for splitting the B−X bond of IM2; in the absence of BX 3 , none of the PES scans performed provided any TS estimate, and the potential energy grew continuously. From this we conclude that the Lewis acidic orbital role of BX 3 is crucial for the leaving halide stability while the electrostatic ion-pair-stabilizing role of BX 3 is only of secondary importance.
The results demonstrate that in the case of chloroboration, unlike for bromo-and iodoboration, the halide-leaving step is endergonic. This is consistent with a weaker Lewis acidity of BCl 3 as compared to BBr 3 and BI 3 . For Gibbs free energy profiles at the B3LYP-D3 and MP2 levels, see Figure S4a−c.
3.2. Polar Additions to Propyne. 3.2.1. syn-Haloboration. Before discussing chemical aspects of propyne synhaloboration, we use its reaction mechanism for a methodcomparison purpose. The reason for choosing this profile in particular is the high methodological (MP2 or B3LYP-D3) sensitivity of the reaction barriers. Unlike for the rest of the mechanisms, method-comparison data in Figure S5 are reported as electronic energies, i.e., they include neither ZPE or thermal contributions to enthalpy nor entropy contributions to Gibbs free energy. The main reason for doing so are high computational demands for obtaining the numerical Hessian at the DLPNO-CCSD(T) level of theory. Figure S5 compares electronic energy profiles of propyne syn-chloro-and syn-bromoboration at the following levels of theory: MP2 electronic energies calculated on MP2-localized stationary-point structures are compared with DLPNO-CCSD(T) energies for the same (i.e., MP2-localized) structures. Similarly, B3LYP-D3 electronic energies calculated on B3LYP-D3-localized stationary points are compared with DLPNO-CCSD(T) energies for B3LYP-D3-localized structures. MP2 total activation barriers, in the sense of highest TS energy with respect to reactants, are overestimated by at most 3 kcal/mol with respect to the CCSD(T) reference. In case of B3LYP, they are underestimated by as much as 7 kcal/mol. Furthermore, MP2 correctly predicts the energies of bromo intermediate IM to lie below those of the neighboring transition states. In contrast, B3LYP predicts the existence of an intermediate for the chloro derivative, but the comparative CCSD(T) calculation shows that IM lies higher in energy than TS1, i.e., that the route from TS2 toward the reactant is barrierless. Based on these results, we consider the MP2/6- 31+G*_SVP method superior to the B3LYP-D3/6-31+G*_SVP metod for the purpose of geometry optimization. CCSD(T) calculations are employed throughout this work for energy evaluation. Comparative MP2 and B3LYP energy data are given in the Supporting Information.
While syn addition to acetylene was (except for one case) a single-step reaction (cf. Figure 1), syn-haloboration of propyne is for most systems and methods a two-step reaction (cf. Figure  4). In terms of halogen influence, the total energy barrier decreases, in agreement with the increasing Lewis acidity of BX 3 . The presence of the additional CH 3 group in propyne apparently strengthens disperse interactions between the halogen lone pairs and the π bonding electrons. As a result, the intermediate IM is (except for two cases) a local minimum on the PES, and the total energy difference between reactants and TS2 is smaller for each propyne haloboration than for the respective acetylene haloboration. For Gibbs free energy profiles at the B3LYP-D3 and MP2 levels of theory, see Figure  S6a−c.

anti-Haloboration
Catalyzed by X − . The first step of propyne anti-haloboration is analogous to its syn counterpart: boron attaches to the less substituted carbon of the triple bond. The structure of the resulting IM1 differs from its acetylene analogue by the additional CH 3 group. Consequently, the approaching halide anion can act not only as a nucleophile but also as a base to abstract one of the CH 3 protons, whose acidity is enhanced by the neighboring unsaturated positively charged carbon (vide infra). The reaction profile for the nucleophilic attack is given in Figure 5, which demonstrates the formation of the anti-haloborated propyne in two steps through the propyne−BX 3 complex IM1. Note the continuously decreasing relative energy of the second transition state, TS2, with    Figure  S8.
To abstract a halide anion from the quaternary boron in IM2, the participation of another molecule of BX 3 is needed ( Figure 6). While the MP2 results depicted in Figure S9 predict Δ r G°to change from +2 to −0.6 kcal/mol in going from X = Cl to X = I, B3LYP-D3 results copy the same trend with slightly more negative values of Δ r G°.

Proton Abstraction from Propyne−BX 3 Complex.
Besides the nucleophilic attack on the propyne−BX 3 complex, X − can also behave as a base that abstracts the methyl proton to provide HX and an allenic intermediate IM3 (Figure 7; for B3LYP-D3 and MP2 results, see Figure S10). Obviously, in the case of Figure 7a,c the Gibbs activation energies are negative from the reactant side. We have therefore augmented Figure 7 with electronic energy profiles, which are already free from negative activation barriers. The improvement is expected to stem from the missing vibration correction inaccuracies. The related less-positive (compared to B3LYP-D3 and MP2) electronic energy activation barriers are expected to originate in MP2 geometry problems for the weakly bound reactant complexes.
In order to explore reaction pathways to the E adduct thoroughly, for the case X − = I − we performed a PES scan for an interaction between the allenic π system of IM3 and a HI molecule with iodine coordinated to C2. As shown in Figure 8 (for MP2 results, see Figure S11), I − can indeed bind to C2 to produce IM5, which is however chemically much distant from the desired E adduct. From this and from the high energy demands involved, we consider the route in Figure 7 a blind alley of anti-haloboration. As noted by a reviewer, BX 3 and a base can give terminal alkyne deprotonation, but deprotonat-ing actually the C−H and not the CH 2 R unit. 57 Therefore, prior to the formation of the IM1 + X − complex of Figure 5, BX 3 and X − can deprotonate the alkyne to provide an alkynyl trifluoroborate, which presents another competing reaction preventing the anti-haloboration. 3 Attack. As discussed recently in our joint experimental and theoretical study of acetylene bromoboration, 12 the most straightforward radical mechanism leading to (E)-dihalo(2-halovinyl)borane is BX 3 addition on halovinyl radical. The latter can be formed by adding halogen radical (initially formed by HX homolysis) on acetylene. Our results for reaction Gibbs free energy profiles of (Z)-vinylhalide radical formation are shown in Figure 9. The standard reaction Gibbs free energy is positive for X = I. However, the following reaction with BX 3 consumes the   Figure S14. The attack of the BX 3 molecule on the (Z)-vinyl halide radical is shown in Figure 10. TS1 adopts C s symmetry with one boron−halogen bond of BX 3 eclipsed with the halovinyl unit and with the C β −C α −B angle of 119°for X = Cl and 118°f or X = Br and I. This orientation is indicative of σ attack due to a maximum possible overlap between the sp 2 hybrid orbital of C α and the largely empty p orbital of boron. The latter is symmetrical with respect to the halovinyl plane, which prevents π attack by the C�C bond. The transition state transforms into IM via C−B bond formation and BX 3 unit deformation, which reveals an interesting halogen dependence. While chlorine and bromine bend "toward" the halovinyl unit to create a triangular B−X−C arrangement, iodine bends away. Below we present a molecular orbital analysis of the halogenbridged product formation for the case of X = Br.

Halovinyl Radical Formation and BX
Key orbital interactions underlying Figure 10b are shown in Figure 11. Due to strong spin polarization, the singly occupied molecular orbital (SOMO) is represented by the secondhighest orbital of α spin, 79α. According to the sums of net Mulliken basis function populations, the SOMO is dominated by an in-plane sp 2 hybrid of C α pointing at B and weakly binding H α (34%). Further contributions are from the p orbitals of Br at C β (24%) and from the two out-of-plane BBr 3 bromines (each 10%). In summary, the 79α SOMO is   pair is merely that of a π antibonding interaction between the vinyl bromide and BBr 3 lone pairs. When TS transforms into IM, its structure rearranges as follows: The C α −B distance contracts significantly (from 2.44 to 1.57 Å) while the C�C bond lengthens slightly (from 1.28 to 1.38 Å). The BBr 3 unit undergoes pyramidalization and reorientates, giving rise to a triangular C α −Br−B arrangement with Br located over the newly formed C α −B bond (we denote this Br atom as Br α ). Symmetry breaking enables an intermixing of previously orthogonal MOs, delivering the majority of spin in the SOMO (80α) to the C β end (34% at Br β and 25% at C β ), while 29% is concentrated at Br α and only 8% remains at C α . The (76α, 79β) orbital pair represents a doubly occupied MO with significant bonding character between the vinyl bromide and BBr 3 units. Indeed, even though lone pairs of two "in-plane" BBr 3 bromines dominate in the (76α, 79β) pair of Figure 11 (each contributing with 27%), "parallel" p orbitals of Br α and C α contribute as well (11% and 5%, respectively) and act as a pathway for π back-bonding.
The content of Figure 11 can be related to a literature-based mechanistic view of borane reactivity toward radicals. According to Renaud et al., 13 homolytic substitution at boron does not proceed with C-centered radicals, but heteroatom-centered radicals react efficiently with organoboranes. This difference was attributed to the Lewis base character of the heteroatom-centered radicals, since the first step of the homolytic substitution is the formation of a Lewis acid−base complex. 13 The double bond of the vinyl bromide radical can be considered as an alternative source of a Lewis base character, and the mechanism described in Scheme 3 can be constructed. In terms of TS geometry, our results contradict Scheme 3, which implies a π attack by electrons of the double bond. On the other hand, beyond the TS, π electrons of the C�C bond are symmetry-allowed and do participate in the delocalization from C α to boron (cf. MO 80α of IM). Computed data also agree with Scheme 3 in that they predict the unpaired electron to remain localized at C α in the TS and shift to C β afterward. Finally, Scheme 3 correctly describes the subsequent shift of electron density from the B−Br bond on C α (a π back-donation from Br α to C α ) and a spin density concentration at Br α . In summary, our results agree with Scheme 3 except for a σ-attack step that is missing at the beginning of Scheme 3 but consistent with our computation.  Figure 12b). We have therefore augmented Figure 12 with electronic energy profiles, for which the problem becomes less severe but partially persists. While the improvement is expected to stem from the missing vibration correction inaccuracies, persisting unphysical behavior is expected to originate in MP2 geometry problems for the weakly bound product complexes. Two possible reaction paths are covered: in Figure 12a, halogen leaves from the boron atom in just a loose coordination to the BX 3 molecule, which preserves a closeto-planar geometry. In Figure 12b, a two-step mechanism is followed: halogen first skips to the α-carbon, and from there it leaves as a part of a tetrahedral BBr 4 radical. From the point of view of activation barriers, a direct splitting of halogen from boron is preferred. This endergonic step is again triggered by a coupled propagation reaction in which halogen radical attacks a fresh acetylene molecule. MP2 energy profiles are displayed in Figure S16. Energy profiles for direct splitting of bromine from boron at additional levels of theory are displayed in Figure S17.

Radical Reactions of Propyne Providing
Haloborated Products. Unlike for acetylene, a halogen radical attack on propyne can result in two possible products: a (Z)-2halopropen-1-yl radical and a (Z)-1-halopropen-2-yl radical. Experimentally, however, halogen atom is known to attack the less substituted carbon of the alkyne, which is understood in terms of greater stability of the resulting radical. 58 This textbook rule is illustrated by our results for energy barriers and product stabilities in the case of Cl, Br, and I addition on propyne in Figure 13. Energy barriers for Cl, Br, and I addition on the terminal carbon are in all cases lower than in the case of the central carbon. Additionally, the (Z)-1-halopropen-2-yl radical is in all cases the more stable one. Propyne attack reactions are increasingly endergonic for heavier halogens but are expected to be triggered by radical consumption in further steps of the mechanism. MP2 energy profiles are displayed in Figure S18.
Based on the above-mentioned results, the consecutive reactions of the radical haloboration mechanism were only explored for the (Z)-1-halopropen-2-yl radicals. Our results for reaction Gibbs free energy profiles have been divided into two parts: the first step involving halopropene radical addition on BX 3 (Figure 14; for B3LYP-D3 and MP2 results, see Figure  S19) and the second step involving excessive halogen atom splitting ( Figure 15; for B3LYP-D3 and MP2 results, see Figure S20 and S21). The complete mechanism is summarized in Scheme 4.
The second step of (Z)-1-halopropen-2-yl radical haloboration ( Figure 15) is endergonic but can be triggered by a propagation step in which the BX 4 radical attacks a fresh propyne molecule, producing (Z)-1-halopropen-2-yl and BX 3 . The reaction mechanism proceeds along a similar path as in the case of acetylene haloboration product formation shown in Figure 12. Note, however, that while the intermediates of Figure 12 correspond to a bridgelike arrangement of B−X−C atoms for chloro and bromo derivatives, in IM structures of Figure 15 this is so for bromo and iodo derivatives. For the remaining structures, boron adopts a genuinely tetrahedral coordination.
Obviously, due to the regiochemistry of the initiating halopropenyl radicals, only haloboration products bearing the halogen at the less substituted carbon are obtained from our calculations. However, experimental studies report haloboration adducts bearing the halogen at the more substituted carbon. 3 From this, we conclude that the radical mechanism most probably does not operate in the case of higher alkyne haloboration.

CONCLUSIONS AND OUTLOOK
The mechanism of alkyne haloboration has been studied by means of B3LYP-D3, MP2, and DLPNO-CCSD(T) methods with an emphasis on direct pathways toward the antihaloborated adduct. For X = Cl, Br, and I, interactions between BX 3 and acetylene or propyne with and without an additional X − anion, as well as between BX 3 and halovinyl or (Z)-1-halopropen-2-yl radical, have been modeled. A bimolecular reaction between acetylene or propyne and the BX 3 molecule produces the Z adduct through a four-center transition state in a single-step process.
In the presence of X − , acetylene and BX 3 form the E adduct in a two-step sequence. First, BX 3 adds to the triple bond to give a zwitterion whose positively charged carbon is attacked  by the halide anion, at a total cost of 7−9 kcal/mol. In the second step, the "excessive" halide anion from the BX 3 − group leaves with another solvent molecule under mild energy requirements of 2−3 kcal/mol. If BX 3 , in the presence of X − , is added not to acetylene but to propyne, the first step differs only in a substantial decrease in activation barriers for chloro and iodo derivatives. The process is essentially barrierless. Two possibilities for the zwitterion reactivity follow: X − can attack the positive carbon as a nucleophile or as a base. The former option continues to the E adduct through two steps, of which the second requires additional BX 3 molecule participation and is endergonic. The latter option provides an allenic moiety in a single exergonic step. Thus, the polar way of propyne antihaloboration is unlikely due to competitive allene generation.
A similar stereochemical message as for acetylene versus propyne in polar mechanisms is given by free radical mechanisms. Halogen radical attack of acetylene produces (Z)-2-vinyl halide radical, which subsequently reacts with BX 3 to provide a bromine-bridged intermediate. The E adduct is then formed via splitting of the excessive halogen radical by binding it on a second BX 3 molecule (or possibly on acetylene). When it comes to propyne, the only initiation process compatible with experimental and computed data is (Z)-1-halopropen-2-yl radical formation. Upon BX 3 addition, it can provide an anti-haloboration adduct but with regioselectivity different from that reported in experimental studies. Since the radical route for the propyne anti-haloboration is excluded and the polar mechanism is unlikely to proceed, it can be expected that the Z/E mixtures reported in experiments for some higher alkynes result from Z/E isomerization rather than from anti addition.
On the other hand, both polar and radical mechanisms are likely to produce E-haloborated adducts in the case of acetylene. Radical activation barriers for the rate-determining step are ca. 2.5 times lower, which along with experimental results in ref 12 indicates radical formation of (E)-dibromo(2bromovinyl)borane. Important points made by a reviewer should be considered in potential follow-up work. In the presence of excess BX 3 , the concentration of free halide in solutions of BBr 3 will be very low. Thus, the [BX 4 ] − anion could be the source of halide for anti-haloboration. Assessing the energetic feasibility of antihaloboration upon [BX 4 ] − (instead of X − ) catalysis is beyond the scope of the current study but definitly deserves to be explored in future studies.